Screening device for electronic subassemblies on a printed circuit board

ABSTRACT

The invention relates to a screening device comprising: a screening cover, which covers an electronic circuit located on a printed circuit board and comprises an edge that is separated from the component side of the printed circuit board by a gap and a contact device, which is located in the gap and produces an electric connection between the screening cover and a conductive contour element on the printed circuit board. According to the invention, lugs are formed on the edge of the screening cover, said lugs fixing the screening cover on the printed circuit board and maintaining an elastic pretension on the contact device. The contact device is configured as an elastic sealing body that extends continuously through the gap and absorbs electromagnetic waves.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2003/005182, filed May 16, 2003 and claims the benefitthereof. The International Application claims the benefits of Germanapplication No. 10231145.5 filed Jul. 10, 2002, both applications areincorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention relates to a screening or shielding device with ascreening cover which covers an electronic circuit located on a printedcircuit board, with an edge that is separated from the component side ofthe printed circuit board by a gap and a contact device which is locatedin the gap and produces an electrical connection between the screeningcover and a conducting contour element on the printed circuit board,with lugs being formed on the edge of the screening cover by which thescreening cover is fixed to the printed circuit board and maintaining anelastic pretension on the screening device.

BACKGROUND OF THE INVENTION

In electronics there is frequently a requirement for diminishingelectrical or magnetic fields within or outside a specific area.Screening housings which attenuate the emission or entry ofelectromagnetic interference. On printed circuit boards the interferenceradiation can also be caused by subassemblies located on the sameprinted circuit board. With miniaturized subassembles, for example thoseusing SMD technology, interference sinks and interference sources areoften immediately adjacent to one another. For screening field valueswhich change over time small designs of multipart screening devices areemployed.

They mostly consist of a screening frame subdivided into individualchambers by partition walls and soldered to the printed circuit board.To keep the subassembles covered by the screening accessible for testpurposes the screening frame is closed off by a removable cover. Sincethe frame part only becomes mechanically stable once it is soldered tothe printed circuit board it is necessary, when fitting theseminiaturized screening devices, to handle the frame part and the coverpart together. Putting the cover on and taking it off is a cumbersomeprocess and assembly is time-consuming.

A single-part metallic screening device which can be fitted onto aprinted circuit board is known from DE 29 808 620 U1. It is attached bylocking lugs which grip the back of the printed circuit board. Masscontacting of the screen with the printed circuit board is provided by aplurality of spring-loaded tongues. These tongues are under elasticpretension once a screening tray is assembled and make electricalcontact by line or point contact. Under harsh operating conditions itcan occur that contacting fails as a result of mechanical effects orcorrosion. In this case the screening efficiency of the interferencesuppression medium is adversely affected. A further disadvantage is thata compression force is necessary on assembly of the screening cover toovercome the spring pressure, said force having to be applied to thescreen tray and borne by the printed circuit board. The screening effectdepends on the contacting spring pressure and the distance between theindividual contact points. If however a large number of spring tongueshave to be elastically deformed a correspondingly greater compressionforce is required and it can occur with miniaturized modules that thecircuit board is impermissibly deformed. Micro cracks can then developin the conductive structure.

Contact is also established by contact springs around the edge of thescreening in the screening device known from DE 297 13 412 U1.

Manufacturing these known one-piece screening devices requires a complexmanufacturing tool. Changes to this tool required to produce a newscreening geometry or spatial form are only possible with acomparatively large amount of effort.

SUMMARY OF THE INVENTION

The underlying object of the invention is to develop a screening deviceof the type stated at the start which improves screening efficiency andmakes it possible to manufacture screening devices, especially those ofdifferent sizes, with less effort.

In accordance with the invention this object is achieved by the claims.The dependent claims make reference to advantageous embodiments.

In the inventive screening device there is provision for the contactingdevice to be formed by a sealing element running in a gap around theedge of the unit, with the sealing element being made of an elasticmaterial which absorbs sources of electromagnetic waves. Characteristicof the invention is also the fact that linear or point contacting is notestablished by a plurality of contact points but by a sealing elementmade of elastic material lying on the surface of the board. Because ofits compressibility this is better adapted to undulations and unevennessin the printed circuit board. Even with mechanical shocks or vibrationsa lower contact resistance of ground contacting is maintained. Inaddition the sealing element running around the circumference in the gapbetween screening housing and circuit board forms a seal and in this wayalso protects the interior from contamination penetrating into thescreening.

The constructional separation of the contacting and the screening framemakes it easier to manufacture. The tool for manufacturing the screeningcover is a simpler design. Upgrade costs for changes to themanufacturing tool are lower. The screening cover can be manufactured atlow costs as a punched-bent part from sheet metal. The sealing elementcan be a blank or a semi finished product. Overall manufacturing can beadapted to a partial screening of different-sized areas on a circuitboard with little effort.

Surprisingly it has transpired that an EMF seal which is just a fewmillimeters wide provides very efficient screening against RF fields.

Materials which have an attenuating effect on electromagnetic radiationare known in various compositions and versions and are commerciallyavailable.

For easy attachment of the screening cover to the circuit board there isprovision for the circuit board to be provided with openings and for anend of a lug projecting through the opening to be elastically deformedso that it grips the back of the printed circuit board. This produces atight-fitting connection which is easy to establish. The plasticdeformation of the projecting end sections can be formed by bending,stamping or pressing. To enable the cover to be easily removed for testpurposes each end part of a lug can be embodied as a hinged flap.

As regards the manufacturing costs it is useful for the screening coverto be formed from a uniform material and in one piece from a metallicmaterial, for example from sheet metal. The sheet metal can be protectedby tin plating.

In a further embodiment of the invention there is provision for the edgeof the cover to be embodied as a right-angled folded edge which, in theassembled state of the screening cover essentially runs parallel to thecomponent side and for each lug to be formed on the outsidecircumference and to be embodied offset from a wall of the screeningcover. This enables the screening cover to be manufactured using apunching or bending process.

With this punched-bent part a very effective screening cover can beimplemented by embodying the sealing element as a flat seal andattaching it by electrically-conductive adhesive to the edge of thescreening cover or the component side of the circuit board. Thisproduces the additional advantage of enabling the screening cover alongwith the seal to be handled as one piece during assembly.

To improve contact reliability the conducting contour of the circuitboard features domed contact points which press into the flat seal. Itssurface is sealed by the elastic flat seal and largely protected againstcorrosion.

If only partial areas on a printed circuit board need to be screened itcan be useful for the screening cover to be embodied in a rectangularshape to match the pitch of the printed circuit board as regards thegeometry. This uniform modular construction reduces manufacturing andlogistics costs.

For the case where the interior of the screening cover has to beventilated or vented there is provision for holes to be provided in thetop or the side walls of the screening cover to form openings forcooling air. The size of the holes is adapted to the frequency spectrumto be screened.

If part areas with different screening effects are required on thecircuit board there is provision for a number of screening covers to bearranged on the printed circuit board and for the screening efficiencyof these covers to differ.

To further reduce costs, commercially-available EMC materials can beused, that is polymer materials, the particular preference being forpolyamide fleece which has a metal coating or is enclosed by a metalmesh.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained below in greater detail on the basis of anexemplary embodiment shown in the drawings. The drawings show:

FIG. 1 a perspective exploded diagram of the inventive screening device,in a embodiment in which the entire printed circuit board is covered bythe screening cover,

FIG. 2 an enlarged detailed diagram of the screening cover from below,

FIG. 3 a detailed cross-sectional view of the edge area of a screeningdevice mounted on a printed circuit board,

FIG. 4 a section of the conducting contour on the printed circuit boardwith contact points arranged offset at intervals.

DETAILED DESCRIPTION OF THE INVENTION

In the drawings FIG. 1 shows a typical embodiment of the inventivescreening device 1 in an exploded perspective view. The screening device1 consists of a screening cover 20 and a contact device 6. In theexample shown, the screening cover 20 covers the entire area of theprinted circuit board 2. The explanations below are not howeverrestricted to this embodiment but in particular also include screeningdevices which cover only parts of the printed circuit board.

As shown in FIG. 1 lugs 8 are formed on the outer edge 3 of thescreening cover by which the screening cover 20 can be attached to acomponent side 4 of the printed circuit board 2. On the component side 4of the printed circuit board 2 a guide contour 7 can be seen of whichthe outline corresponds to the edge area of the screening cover 20. Theguide contour 7 consists of contact points which are explained ingreater detail below and is etched to correspond to the pitch of theboard. The contact device 6 is embodied as a seal 22 running around theedge. When the screening device is assembled the screening cover 20 islowered onto the printed circuit board 2. As the cover is lowered ontothe board, lugs 8 pointing in the direction of the printed circuit board2 engage in openings 10 of the circuit board As the lowering operationproceeds the elastic sealing element 22 is initially compressed. Becauseof the good compression characteristics of the seal material only alight force is required to compress the seal. Lowering the cover furthercauses the end 9 of the lug 8 to pass through the opening and projectbeyond the back of the board. To attach the screening cover the lugs nowprojecting from the opening on the exit side are plastically deformed bybending them on both sides. A tight-fitting connection is made betweencover 20 and carrier 2. As already shown, the plastic deformation canhowever also be made by bending, by stamping or by pressing. In the gapbetween the edge of the cover and the component side the elastic sealingelement 22 will be compressed. The electromagnetic radiation energypenetrating into the sealing gap 5 is greatly attenuated as a result ofthe damping material properties of the sealing material. Because thesealing material is only slightly compressed assembly requires only acomparatively light pressure.

FIG. 1 shows the screening cover with two preformed attachment lugs onits long side and one lug on its short side. Of course the number ofattachment lugs varies in accordance with the size of the screeningcover. The openings 15 for ventilation or venting of the interior areshown greatly enlarged. The diameter of these holes is in reality verysmall and tuned to the frequency of the alternating field to bescreened. Screening cover 20 is a one-piece bent-punched element madefrom tin-plated sheet metal.

FIGS. 2 and 3 show the edge area of the screening cover in an enlargeddetailed view. The edge 3 of the screening cover 20 is continued by aright-angled fold 12. On the outside circumference the lug 8 is formedon the fold 12. The end section 9 of the lug is embodied as a hingedflap 11. As can be easily seen from the cross-sectional diagram shown inFIG. 3, the lug 8 is inserted through the opening 10 of the printedcircuit board 2. The end section 9 is plastically deformed along theaxis pointing in the direction of insertion by bending on both sides. Inthis way the hinged flap 11 grips the back of the printed circuit board2. With a closed embodiment of the screening cover the seal runningaround the edge 22 also simultaneously protects the interior 18 againstdust or contamination entering from the outside 19. In thecross-sectional view shown in FIG. 3 the seal 22 is shown as a flat seal13. It is located in the sealing gap 5 between the printed circuit board2 and the edge 12 under elastic pretension. The flat seal 13 is coatedon its upper side with an adhesive 14 which means that seal andscreening cover form a single unit for assembly and are easier tohandle. The underside of the flat seal 13 lies along the conductingcontour 7 on the component side 4. The conducting contour 7 is formed bystepped contact points 21 linked electrically by a corresponding layout.Compressing the seal in the gap 5 causes the seal material to deformelastically. The contact points 21 press on the underside of the flatseal This enclosure of the domed surface of the contact point means thatit is very well protected from outside influences. This counters theeffects of corrosion in the contact area. The result is that over a verylong period of use the ohmic resistance of the ground contacting can bekept very low. Contour 7 can also be formed by a continuous conductortrack or by another contact pattern. Naturally it is also possible tocoat the other side of the flat seal 13 with a conductive adhesive.

The arrangement of the dome-shaped contacts 21 along the conductingcontour 7 is shown in an enlarged view in FIG. 4. The contact points 21are arranged staggered at intervals. The gap between the contact pointsin the lengthwise direction of the mass contacting is four millimetersin the example. The domes on the contact points are tinned and have adiameter of 1.3 millimeters. The EMC flat seal is made of a polyamidespun bond fleece which has a high compressibility of up to 85 percentand is very flexible.

The EMC seal can however be constructed in a different way and made ofother materials. The seal can for example be an open-cell foam plasticto which electrically and magnetically conductive particles are added.Woven and compound materials are also suitable which contain fibers of amaterial with these conductivity properties.

The metal mesh can be a tinned, copper-coated steel wire mesh. Thesealing element can be braided by the steel mesh. It is also possiblehowever for the metallic mesh to be embedded into the sealing element bypolymer attachment.

1. A screening device, comprising: a screening cover, which covers anelectronic subassembly arranged on a printed circuit board, with an edgewhich is spaced from a component side of the circuit board by a gap; anda contact device which is arranged in the gap and establishes anelectrical connection between the screening cover and a conductingcontour on the circuit board, wherein lugs are formed on the edge of thescreening cover by which the screening cover is attached to the circuitboard and the contact device is held under elastic pretension, whereinthe contact device being embodied as an elastic sealing element runningaround the circumference of the gap and absorbing electromagnetic waves,and wherein the conducting contour is formed by dome-shaped contactpoints.
 2. The screening device in accordance with claim 1, wherein thedome-shaped contact points are arranged on the component side of theprinted circuit board at the printed circuit board pitch, or at a griddimension of the printed circuit board, or at a lead-wire spacing of theprinted circuit board.
 3. The screening device in accordance with claim1, wherein the printed circuit board further comprises openings throughwhich the lugs of the screening cover protrude on the exit side andwherein plastically deformed end sections of the lugs grip the back ofthe printed circuit board.
 4. The screening device in accordance withclaim 2, wherein the printed circuit board further comprises openingsthrough which the lugs of the screening cover protrude on the exit sideand wherein plastically deformed end sections of the lugs grip the backof the printed circuit board.
 5. The screening device in accordance withclaim 3, wherein each end section of a lug is designed as a hinged flapor a twist-lock flap.
 6. The screening device in accordance with claim1, wherein the screening cover is embodied in a uniform material and inone piece and is made of metal.
 7. The screening device in accordancewith claim 2, wherein the screening cover is embodied in a uniformmaterial and in one piece and is made of metal.
 8. The screening devicein accordance with claim 1, wherein the screening cover is designed in auniform material, merged into each of its elements, and is made of ametallic material.
 9. The screening device in accordance with claim 1,wherein the edge is designed as a right-angled fold running in theassembled state of the screening cover essentially parallel to thecomponent side, and wherein each lug is formed on the outsidecircumference and is embodied offset along a wall of the screeningcover.
 10. The screening device in accordance with claim 2, wherein theedge is designed as a right-angled fold running in the assembled stateof the screening cover essentially parallel to the component side, andwherein each lug is formed on the outside circumference and is embodiedoffset along a wall of the screening cover.
 11. The screening device inaccordance with claim 1, wherein the sealing element is designed as aflat seal and is attached by an electrically conductive adhesive at theedge of the screening cover or the component side.
 12. The screeningdevice in accordance with claim 2, wherein the sealing element isdesigned as a flat seal and is attached by an electrically conductiveadhesive at the edge of the screening cover or the component side. 13.The screening device in accordance with claim 1, wherein the screeningcover is embodied as a punched-bent part.
 14. The screening device inaccordance with claim 2, wherein the screening cover is embodied as apunched-bent part.
 15. The screening device in accordance with claim 1,wherein the screening cover is designed in a cuboidal or rectangleshape.
 16. The screening device in accordance with claim 1, whereincutouts are provided on a top and/or a side wall of the screening cover.17. The screening device in accordance with claim 1, wherein a pluralityof screening covers are arranged on a printed circuit board and thescreening efficiency of these screen covers is different.
 18. Thescreening device in accordance with claim 1, wherein the sealing elementis formed from a polymer material metallically coated or surrounded by ametallic mesh.
 19. The screening device in accordance with claim 18,wherein the polymer material is a polyamide weave or fleece.
 20. Thescreening device in accordance with claim 1, wherein the sealing elementis formed from an electrically conductive elastomer braided by a tinnedcopper-coated steel wire mesh.